For many years, commercial water heaters have been constructed using burners and heat exchanger water flow tubing. Commercial water heaters must be capable of producing and heating water with tens of thousands, and even hundreds of thousands, of BTUs. Further, in modern commercial applications, the emission standards for water heaters are strictly regulated. Complete burning of fuel is controlled so that hydrocarbon emissions are very low. In many existing commercial water heaters, natural gas is burned in an environment of forced air.
Many direct-fired, commercial water heating systems are known in the industry. One commercially available system, disclosed in U.S. Pat. No. 4,261,299, utilizes a horizontal combustion chamber around which water flows through a double-walled shell that is wound repeatedly around the combustion chamber with spaces between each successive winding to accommodate a countercurrent flow of exhaust gases.
Another system, disclosed in U.S. Pat. No. 4,938,204, utilizes a dual tank design. One tank contains the primary heat exchanger in which a horizontally mounted conventional burner heats water flowing through two-pass, U-bend fire tubes. Exhaust gases that exit the primary heat exchanger at 350 degrees Fahrenheit to 400 degrees Fahrenheit are routed to a secondary heat exchanger where they are passed countercurrent to ambient makeup water to preheat the water before entering the primary exchanger. Makeup air is preheated to over 200 degrees Fahrenheit by passing it through ductwork which surrounds the exhaust gases exiting the secondary exchanger.
Some of the newer prior art systems utilize primary exchanger sections comprising a vertically-disposed, radially-directed, cylindrical burner in combination with a plurality of fixed length, copper-finned tubes arranged vertically around the burner. Water flows through the tubes, which are typically connected to headers located above and below the combustion zone, either in single or double-pass configurations. In some heaters, the copper-finned tubes are intermeshed and completely surround the burner to enhance heat transfer. Difficulties have been experienced with these heaters, however, because of the length of the tubing required to allow for effective heat exchange and the limited amount of expansion or contraction that can be accommodated with the fixed tube design.
U.S. Pat. No. 5,687,678 discloses a commercial water heater apparatus, including a housing, a radial-fired burner within the housing, a single continuous, multiple-loop, finned coil tubing heat exchanger for circulating water around the burner, having at least a first set of inner coils forming a coil trough therebetween and a second set of outer coils nested within the coil trough formed by the inner set of coils, the outer set of coils forming a second coil trough around the exterior thereof, and a coil baffle interposed in the second exterior trough for deflecting heat adjacent to the second set of coils.
Highly efficient transfer of heat energy from the burned fuel to the water has been an object of commercial water heater design for a number of years. In accomplishing the high efficiency heat transfer from the combustion products to the circulated water, in many systems a certain amount of water vapor in the combustion gases will be condensed from the combustion gas. This condensate is typically highly acidic, having PH values in the range of between 2 to 5, depending upon the chemical constituents of halogenated hydrocarbon in the natural gas and air mixture. For example, increased halogen content of the natural gas and air mixture can greatly increase the acidity of the condensate. Therefore, various commercial water heaters are simply designed to operate below the efficiency at which large quantities of condensate are likely to form so that the acidic vapors are discharged in vapor form in high temperature exhaust gas.
Notwithstanding the systems disclosed in the prior art, it would be beneficial to have a radial-fired heat exchanging apparatus which has a compact configuration and which can quickly and efficiently transfer heat to water passing through the tubes.